答案：D

解析：

Choice D is correct. The goal is to find the probability that a day in spring was either partly cloudy or cloudy. Written as a ratio, the probability is

Probability=(#of days in the spring that were partly cloudy or cloudy)/(of days in spring)

Since the number of days in the spring that were partly cloudy is 25, the number of days in the spring that were cloudy is 15, and the total number of days in spring is 91,

Probability=(25+15)/91=0.439

Multiplying 0.439 by 100 gives approximately 44%.